As used herein, a physical channel is the frequency used in a system. Whereas, logical channels are subdivided from the physical channel. The main attraction for any full-duplex two-way radio system is the ability for the listener to interrupt and speak at any time. Conventional two-way radio systems operating in the duplex mode require two different frequencies for communciation to take place in both directions simultaneously; this wastes spectrum. Others have simulated duplex operation on a single physical or logical channel by utilizing hardware control of the transmitter and receiver and sampling the physical or logical channel for the presence of a carrier wave. This approach, however, generally requires increased radio complexity.
Additional problems arise, however, when sufficient bandwidth is not available. For example, the current state of the art in land mobile communications provides for a maximum channel bit rate of approximately 16 kb/s on 25 kHz spaced radio frequency physical channels. If 10% of the channel bit rate is allotted for synchronization and control, 14.4 kb/s is left to be equally divided between two transmitters. However, 7.2 kb/s is not sufficient assuming that 10.8 kb/s is necessary for a good quality voice signal. Therefore, a need exists for a method to provide efficient duplex operation on narrowband land-mobile communication physical or logical channels, while maintaining voice quality.
Given sufficient bandwidth in the physical or logical channel, time division multiplexing (TDM) of voice and data onto a single physical or logical channel is known to provide full-duplex radio operation. Generally, the physical or logical channel is divided into frames having at least two time slots for a radio to transmit half the time, and to receive at half the time. While effective, this practice wastes spectrum since, in ordinary conversations, the talker usually receives only brief feedback responses (or acknowledgements) during his or her transmissions.
Even if sufficient bandwidth is available on a wideband physical channel, using less bandwidth while maintaining voice quality allows partitioning the physical channel into more logical channels to efficiently accommodate more users.